How to translate plane coordinates x,y to equirectangular?
It seems to me, that the number, that used in Power node is not exactly equals 2. It can be like 2.0001, but in widget it rounds 2. Why is important? Coordinates are in range -1 ... 1 so part of coordinates are negative. And raising a negative number to a fractional degree is impossible (because of math), so it brakes the resulted circle.

Equals 2:


Does not equal 2: